Optical fiber gratings are an important enabling technology for the realization of many optical sensors for a wide variety of measurements. Due to their many advantages including compactness, high sensitivity, fast response, electromagnetic immunity and multiplexing capability, a variety of sensors such as strain and temperature sensors, refractive index sensors, have been developed by using fiber Bragg gratings (FBGs) or long-period gratings (LPGs). In recent years, researchers have attempted to develop fiber grating sensors to measure different parameters for various industrial applications. This permits the use of a single interrogator to deal with many FBGs to measure various kinds of parameters that could eventually increase the cost effectiveness of FBG sensor systems.
One kind of FBG-based sensor being developed is an optical fiber anemometer or flowmeter for the measurement of gas or liquid flow speed. An FBG-based anemometer has widespread applications, for example in wind turbines where FBG sensors are being actively investigated to measure temperature, strain and vibration of wind turbine blades. Although many kinds of anemometers and flow-meters have been developed using different principles including laser, Doppler technology, cross-correlation technique or fiber modal interferometer, and hot-wire anemometry.
Hot-Wire Anemometry (HWA) is based on the heat transfer from sensors to the surrounding environment. In order to achieve a localized hot fiber section, light-absorption or active fibers can be used for making spectrum controllable FBGs or LPGs. Compared with passive fiber grating sensors, fiber grating sensors heated with laser power have some enhanced features like controllable sensitivity, responsivity, and dynamic range to enable measurement in large temperature range. An FBG anemometer externally heated by using a CO2 laser for measuring gas flow in the high-voltage environment (corona discharge) may be too bulky for some industrial applications. To address this, a flowmeter based on two cross-mounted thin-silver coated FBGs internally heated by laser light for measuring the magnitude and direction of gas flow may be used.
Measurement of wind speed may be performed by localized heating with a CO2 laser beam and use a fiber Bragg grating (FBG) written in an optical fiber to measure convection cooling. The main disadvantages of this approach are: precision alignment of the laser beam to heat the FBG is not practical in many applications, difficulty in ensuring exact amount of light energy absorbed by the FBG; using the temperature-dependent spectrum of FBG to provide light intensity to temperature relationship which is not as reliable, and difficulty in multiplexing several sensors together.
An optical fiber has been used to deliver light energy to heat a temperature-sensitive material attached to the tip of the fiber. However, this system is complicated and difficult to implement. It uses an optical fiber to deliver light to heat an element attached to the tip of the fiber making multiplexing difficult.
A light absorption material (such as epoxy) has been used which is placed at the tip of an optical fiber and heated by a laser beam via the optical fiber. A thermocouple element inserted in the material was used to measure the temperature. It is basically an electrical point sensor and therefore experiences the same disadvantages as any electrical sensor such as being prone to Electromagnetic Interference (EMI) and difficulty in multiplexing.
A fiber Bragg grating (FBG) has been used as a temperature sensor which uses a laser beam delivered via the fiber to heat a thin metallic coating deposited onto the surface of the fiber surrounding the FBG. The main disadvantages are that it is an extrinsic sensor in which metallic coating needs to be deposited onto the fiber surface. It also suffers from inefficient heating and the thin-film heater cook quickly by fluid and therefore may not be suitable for fluid flow measurement.